1. Field
The present disclosure relates to a light-emitting device and a display device.
2. Description of the Related Art
Recent years have seen the development of light-emitting devices that have (i) a light-emitting element such as a light-emitting diode (LED) combined with (ii) a wavelength-converting member that transforms excitation light from the light-emitting element into fluorescence (e.g., particles of a phosphor dispersed in resin). Advantageously compact and consuming less power than incandescent lamps, light-emitting devices of this type are in practical use as light sources for various display devices and illumination devices.
A commonly used light-emitting device of this type is a combination of a blue LED and a yellow phosphor. The yellow phosphor is typically Ce-activated YAG (yttrium aluminum garnet) because of its high luminous efficiency.
When a light-emitting device is used as a display device, the color gamut of the display expands with decreasing width at half maximum of the emission spectrum of the phosphor. However, the width at half maximum of the emission spectrum of Ce-activated YAG is relatively broad, approximately 100 nm. When a light-emitting device that includes Ce-activated YAG as a yellow phosphor is used as a liquid crystal backlight for a display device, therefore, the color gamut will be insufficient.
Specifically, such display devices cover almost the entire sRGB color space, which is the color space for CRTs (cathode ray tubes), but not as much for the NTSC (National Television System Committee) and Adobe RGB color spaces, which are broader than the sRGB and used for wide color gamut liquid crystal displays.
To be more specific, display devices in which a Ce-activated YAG light-emitting device is used as a liquid crystal backlight cover approximately 70% of the NTSC and Adobe RGB color spaces. Such a light-emitting device is therefore not suitable for use in wide color gamut liquid crystal displays.
The sRGB color space is a color space inside the triangle defined by the following three chromaticity points on the CIE (Commission Internationale de l'Eclairage) 1931 color diagram: (CIEx, CIEy)=(0.640, 0.330), (0.300, 0.600), and (0.150, 0.060).
The NTSC color space is a color space inside the triangle defined by the following three chromaticity points on the CIE 1931 color diagram: (CIEx, CIEy)=(0.670, 0.330), (0.210, 0.710), and (0.140,0.080). The Adobe RGB color space is a color space inside the triangle defined by the following three chromaticity points on the CIE 1931 color diagram: (CIEx, CIEy)=(0.640, 0.330), (0.210, 0.710), and (0.150, 0.060). The NTSC and Adobe RGB color spaces have a larger gamut in terms of green than the sRGB color space.
A light-emitting device that can be used as a backlight in wide gamut liquid crystal displays, such as those supporting the NTSC or Adobe RGB, may have a configuration in which two phosphors, green and red, are used. The emission spectrum of these phosphors may have a relatively narrow width at half maximum.
For example, WO 2009/110285 (published on Sep. 11, 2009) discloses a light-emitting device that includes a combination of a Eu-activated β-SiAlON phosphor (a green phosphor) and a Mn4+-activated fluoride complex (a red phosphor). This combination provides a wider color gamut than in the conventional configuration, in which a yellow phosphor is used, when the light-emitting device is used as a component of a display device. This is because the emission spectrum of both of the Eu-activated β-SiAlON phosphor and the Mn4−-activated fluoride complex has a smaller width at half maximum than that of Ce-activated YAG. Specifically, the width at half maximum of the emission spectrum of the Eu-activated β-SiAlON phosphor is 55 nm or less, and that of the emission spectrum of the Mn4+-activated fluoride complex is 10 nm or less.
Japanese Unexamined Patent Application Publication No. 2010-93132 (published on Apr. 22, 2010) discloses a light-emitting device that includes a combination of a Mn-activated γ-AlON phosphor (a green phosphor) and a Mn4−-activated fluoride complex (a red phosphor), which is an example of a configuration capable of a color gamut even broader than that of light-emitting devices according to WO 2009/110285. This publication states that the emission spectrum of the green phosphor has a peak wavelength of 510 nm to 550 nm, with the width at half maximum of the emission spectrum being 55 nm or less (preferably 45 nm or less). The publication also mentions a Mn-activated γ-AlON phosphor whose emission spectrum has a peak wavelength and a width at half maximum of 515 nm and 33 nm, respectively, as a production example of a green phosphor.
Japanese Unexamined Patent Application Publication No. 2009-218422 (published on Sep. 24, 2009) discloses a light-emitting device that includes a Mn-activated oxide or nitride as a green phosphor. Specifically, this publication discloses a light-emitting device that includes a combination of such a green phosphor and a Eu-activated phosphor (a red phosphor). The publication states that the emission spectrum of the green phosphor has a width at half maximum of 40 nm or less. Similar to Japanese Unexamined Patent Application Publication No. 2010-93132, this publication mentions a Mn-activated γ-AlON phosphor whose emission spectrum has a peak wavelength and a width at half maximum of 515 nm and 33 nm, respectively, as a production example of a green phosphor.
Liquid crystal display devices smaller and thinner than before have been deemed useful in recent years. Light-emitting diodes for use as a backlight for liquid crystal display devices will be more beneficial if they not only improve the color reproduction of the liquid crystal display devices but also, as presented in Sharp Technical Data Sheet “SPECIFICATIONS Surface Mount LED GM5Fxxxx10A” (Jul. 24, 2015), are packaged in smaller light-emitting devices in which the package of the light-emitting diodes is 1 mm thick or thinner.
A Mn (Mn2+)-activated γ-AlON phosphor is used in some cases as a green phosphor to improve color reproduction as in, for example, Japanese Unexamined Patent Application Publication Nos. 2010-93132 and 2009-218422. However, Mn2+-activated γ-AlON phosphors are relatively inefficient in absorbing excitation light. For efficient absorption of excitation light, especially in such a small-sized light-emitting device, it may be recommended to increase the amount of the Mn2−-activated γ-AlON phosphor.
However, a Mn2+-activated γ-AlON phosphor used in an excessive quantity may affect the fluidity of the dispersing material. In such a case, the dispenser used to apply the dispersing material in which the Mn2+-activated γ-AlON phosphor is dispersed can clog. Furthermore, light-emitting devices made with such a phosphor may emit light with varying chromaticities. The reduced fluidity of the dispersing material can therefore lead to a lower yield in the production of the device, which in turn can make the device difficult to mass-produce with consistent quality.
The configuration according to WO 2009/110285 includes a Eu-activated β-SiAlON phosphor, rather than a Mn2+-activated γ-AlON phosphor, as a green phosphor. The Sharp technical data sheet appears to be silent regarding what kind of green phosphor is used.
Japanese Unexamined Patent Application Publication Nos. 2010-93132 and 2009-218422, which disclose Mn-activated γ-AlON phosphors as green phosphors, appear not to disclose or discuss countermeasures against the aforementioned decrease in the fluidity of dispersing material that can occur in small light-emitting devices.
It would therefore be desirable to provide a light-emitting device that includes a Mn2+-activated γ-AlON phosphor as a green phosphor but can be produced with limited loss of yield and to provide a display device that includes such a light-emitting device.